Regional economic strength and the demise of several competing freight haulers helped Southeastern truck lines post strong revenue and earnings gains in the first quarter.

Six of the area's leading regional trucking companies, including AAA Cooper Transportation Co., Southeastern Freight Lines Inc. and Saia Motor Freight Line Inc., posted sharp- ly higher profits during the normally slow first quarter.Generally, regional trucking companies in other parts of the country had trouble matching last year's results.

"The geographic nature of the Sun Belt and the growth that's going on down here is part of the reason," said Mack Dove, president of AAA Cooper in Dothan, Ala.

The shutdown of two large regional trucking companies, Brown Transportation Co. in December and Bowman Transportation Co. in January, also made more business available.

"It's hard to point to specific accounts that we got when those two companies closed," said W.T. Cassels, president of Southeastern. "But you'd have to think that we did get some business."

The only Southeastern truck line that did not report a year-over-year profit improvement, Estes Express Lines Inc., Richmond, Va., still had the best operating margin of any trucking company in the group.

The seven Southeastern companies are among a group of 12 truck lines profiled in the adjoining tables. The Journal of Commerce reviews the

financial and operating results of the nation's top trucking companies each quarter.

The shutdowns at Brown and Bowman did not produce a sudden surge in business for most of the regional compa- nies.

"Our growth over the last three years has been due, in part, to the beginning and ensuing demise of competitors," Mr. Dove said.

Two other regional companies, Steve D. Thompson Trucking Co. and Pilot Freight Carriers Inc., also failed last year.

Smith's Transfer Corp., another major Southeastern carrier, closed in 1988, just months after it was acquired by defunct American Freight System Inc.

Beyond the tonnage gains - three of the seven Southeastern companies reported increases of more than 10 percent - the competitive shakeout seems to have resulted in higher rates for the survivors.

Revenue-per-ton, which gives some indication of rate levels, rose between 6 percent and 12 percent year-over-year for most of the Southeastern companies in the group.

By comparison, major national companies such as Yellow Freight System Inc., Roadway Express Inc. and Consolidated Freightways Inc. reported revenue- per-ton increases of less than 2 percent.

Arkansas Freightways Inc. of Harrison, Ark., posted the strongest overall

gains in the group: Revenue rose 46.4 percent, operating income 100 percent and tonnage 33.7 percent.

Most Southern trucking companies are non-union, which accounts for some of the earnings disparity compared to Northern and Midwestern fleets. But rates in the South for trips of comparable length generally are at least 15 percent lower than in other parts of the country.